PROJECT SUMMARY Development of non-invasive tools for activating deep brain structures is critical for causally manipulating neural function in humans. Furthermore, such method, if able to elicit long-term plastic changes in neural circuits, will aid in functional recovery of neural function. One of the promising non-invasive neural modulation technique that has a potential to activate deep brain structures in a focal manner is ultrasound. Several groups have demonstrated that ultrasound can lead to neural activation, alter sensory responses, or cause behavioral outcomes. In this proposal, we aim to fill a gap in knowledge as to whether focal stimulation of deep brain structures using Low Intensity Focused Ultrasound (LIFU) leads to long-term changes in neural function relevant for functional recovery, especially in the adult brain with limited capacity for plasticity. It is known that the developmental loss of thalamocortical (TC) plasticity precedes the closure of the critical period for cortical plasticity in sensory cortices, which suggests that recovery of TC plasticity may be needed to restore plasticity in the adult brain. In line with this idea, several studies have reported that recovery of adult cortical plasticity is often accompanied by restoration of TC plasticity. A previous study demonstrated that patterned electrical stimulation of the visual thalamus (dLGN) produces long-term potentiation (LTP) of TC inputs to the primary visual cortex (V1) in adult rats. Here we will investigate whether non-invasive LIFU stimulation of dLGN can produce TC plasticity in adult V1. In Aim 1, we will determine whether LIFU stimulation leads to long-term plastic changes of dLGN inputs to layer 4 (L4) of adult V1. To do this, we will use a LIFU stimulation device developed by the Applied Physics Laboratory (APL) at Johns Hopkins University, and combine this with genetic tools to express exogenous genes specifically in activated dLGN neurons. Specifically, we will use genetic methods that can drive the expression of optogenetic tools (i.e. channelrhodopsin-2) selectively to LIFU-stimulated neurons to functionally assess long-term synaptic plasticity, and test the utility of a novel sonogenetic tool that can provide cell-type specificity to LIFU stimulation. In Aim 2, we will investigate whether LIFU stimulation can alter neural response properties of V1 L4 neurons using in vivo 2-photon Ca2+ imaging. Results from our study will determine whether LIFU stimulation can produce long- term plasticity of neural circuits in the adult brain, which can be relevant for designing non-invasive methods for functional recovery. At the very least, our study will provide genetic methodologies that can drive exogenous gene expression in deep brain structures using LIFU stimulation, and will provide information on whether sonogenetics can produce cell-type specific activation of deep brain structures.